Screws made of a synthetic resin are not only remarkably light in weight in comparison with screws made of a metal and can be mass-produced, but also their production cost is relatively low because their material is relatively inexpensive. In addition, they can be easily produced with different pitches and/or different thread shapes in section. Therefore, they have been increasingly widely utilized as being incorporated in various feeding devices.
For example, paper sorters operatively associated with copying machines are required to incorporate such a mechanism that can move a number of vertically disposed sorter trays successively to their prescribed positions so as to bring them to be in spaced arrangement or to be collected at a bottom position, or can broaden the spacing between adjacent sorter trays on the side of the copying machine at which copied papers are discharged than on the other side, and synthetic resin-made screws are utilized in this mechanism. Synthetic resin-made screws are also used in various devices, such as the zoom mechanisms of cameras or the head transporting mechanisms of printers.
If upper and lower molding blocks are used to mold such a synthetic resin screw by means of a cavity the center axis of which lies in a parting plane formed between the upper and lower molding blocks, it will be difficult to withdraw a molded screw longer than one pitch from a mold assembly. Specifically, if the molded screw is observed from one side thereof along its horizontal axis, by noting either one of the side faces (flights) of a particular thread, it can be seen that a top half which corresponds to an angular extent of 90.degree. (a 1/4 pitch of the thread) above the center line is visibly exposed, i.e., the top half is in the "non-undercut state" in which the molded product can be withdrawn from the mold assembly in the direction of the eyes of the viewer. Further, it will be seen that a bottom half which follows the top half and corresponded to an angular extent of 90.degree. below the center line is not visibly exposed, i.e., the bottom half is in the "undercut state" in which no molded product can be withdrawn from the mold assembly.
Further, another top-half side face which is opposite to the non-undercut side face of the thread above the center line is in the undercut state. By noting the opposite side faces of two adjacent threads formed on the opposite sides of a particular root, it will be seen that one of the side faces is in the non-undercut state, whereas the other side face is in the undercut state.
As will be understood from the above description, in each one-pitch portion of a screw thread, a so-called undercut portion which hinders a molded product from being withdrawn from a mold assembly is formed every 90.degree. along either side face of the screw thread. In addition, if either side face is in the non-undercut state, the opposite side face is in the undercut state.
The shape of the screw thread is formed as a twisted plane in the above-described manner. A molded screw having thin threads made of a soft molding material, such as polyethylene resin, can forcedly be released from a mold assembly, but a molded product made of a resin suitable for use as a structure material, such as nylon, cannot be withdrawn from the mold assembly because an undercut portion of the molded product will be caught in the cavity of the mold assembly. Accordingly, it is substantially difficult to carry out molding with a mold assembly having a simple structure made up of a small number of movable members.
Another problem is involved in the cross-sectional shape of a screw thread, such as a square thread or a wedge thread. For example, if a square-threaded screw is molded, the thread is formed in such a manner that each side face of the thread erects at right angles to the adjacent root, so that a major part of the screw thread is formed in the undercut state and it is difficult to withdraw the molded screw from the mold assembly.
Since a withdrawing slope is needed to cope with this problem, it has been necessary to employ a screw with a wedge thread having considerably inclined side faces. If a square-threaded screw having side faces formed at right angles to the root is used in a feeding mechanism, it is possible to realize the function of accurately feeding a feeding member meshed with the square-threaded screw. However, if the side faces of the screw are inclined in the above-noted manner so that the molded screw can easily be withdrawn from the mold assembly, the distance over which a member to be fed is transported becomes inaccurate, accordingly.
For this reason, when a square-threaded screw made of a synthetic resin is to be manufactured, it is necessary to mold a screw having a thread with a trapezoidal cross-section with greatly inclined withdrawing slopes and thereafter form the molded screw into a square-threaded screw by machining. Such a synthetic resin-made square-threaded screw subjected to molding followed by machining can realize a good feeding precision, but it has been impossible to avoid the problem that manufacturing cost becomes higher.
If a synthetic resin-made screw is to be molded by the prior art, because of the technical problem that it is difficult to withdraw a screw from a mold assembly after the screw has been molded, the screw is molded in the state of being cut into "screw units" each having a length corresponding to one pitch with its screw axis positioned in a direction perpendicular to a joining plane of the mold assembly. Then, a projection and a recess are provided in the central portion of each of the screw units so that the projection of each of the screw units can be fitted into the recess of the next one in a socket-and-spigot relationship. Further, to prevent a positional deviation of the screw units, a projection which serves as a stopper and a recess into which to fit the projection are provided in the central portion of each of the screw units.
Incidentally, the "joining plane" of the mold assembly represents the plane in which a plurality of molding blocks join together. When the molding blocks are to move away from each other after the completion of molding, the joining plane serves as a parting plane and appears as a parting line on a molded product. These parting plane and line are hereinafter referred to as "parting plane".
When the thus-molded screw units are to be combined to form a screw of predetermined length, a metal rod is inserted through the holes formed in the central portions of the respective screw units, and to impart strength to the screw, the projection formed on the contact face of each of the screw units is fitted into the recess formed in the contact face of the adjacent one.
However, the synthetic resin screw constructed of the screw units connected in this manner has the problem that since the screw units are separate components each of which corresponds to one pitch of the screw thread, steps are easily formed at the joint faces between the respective connected screw units. If a screw having the steps is employed, no smooth feeding is effected and noise easily occurs. In addition, accurate and stable feeding cannot be effected.
Since the conventional, synthetic resin screw is formed by connecting a multiplicity of screw units in series, dimensional errors are accumulated and amount to a considerably large error with respect to the entire length of the screw. Further, since the screw units each of which corresponds to one pitch are connected by fitting along the axis, the obtained screw is low in mechanical strength and can be utilized in a feeding device for a supporting plate of a sorter of a copying machine at the best.
Although the zoom mechanism of a camera lens or the head transporting mechanism of a printer requires precision screws, it is necessary to employ screws each of which is produced by machining.